1. Field of the Invention
This invention relates to the polymerization of the elastomeric homopolymer polyisobutylene rubber and the elastomeric copolymer butyl rubber, especially the polymerization reaction required to produce the isobutylene-isoprene form of butyl rubber. More particularly, the invention relates to a method of protecting the polymerization process used to produce polyisobutylene and butyl rubbers against molecular weight depression and other undesirable effects caused by the presence of polymerization poisons and/or activators, through addition of a buffering neutralizing or complexing agent to the polymerization zone.
2. Description of the Prior Art
A significant problem encountered in the butyl rubber producing process is the sensitivity of the reaction system to the presence of polymerization poisons and catalyst activators, which, even in trace quantities can substantially adversely affect the polymerization process by reacting or complexing with the catalyst species or propagating carbonium ions to profoundly affect the initiation, propagation, transfer, and termination steps of the polymerization. The consequences are observed as severe molecular weight depression in the butyl rubber product, activation or deactivation of the catalyst system, and often times rapid warmup and plugging of the polymerization reactors. The polymerization poisons and activators include oxygenated compounds such as alcohols, ketones and ethers; acidic compounds such as hydrogen chloride, chlorine, chlorine containing organic compounds, and organic acids. Many of these polymerization poisons or activators are produced in the various zones of the butyl rubber production system where water is present and result from hydrolytic reactions involving methyl chloride and/or methylene chloride. Some are produced by chemical reactions which take place in the alumina drying step of the process which is used to remove water in the diluent recycle section of a butyl plant. Still others are introduced as impurities present in the monomer feed streams. Specific examples of polymerization poisons and/or activators include methanol, isopropanol, acetone, dimethylether, diethylether, dimethoxymethane, t-butyl alcohol, t-butyl chloride, bis-chloromethylether, gaseous chlorine, hydrogen chloride, propionic acid, and the like.
Recognition of this problem in the prior art is found in U.S. Pat. No. 3,005,008, issued Oct. 24, 1961, to kelley et al wherein polymerization poisons are removed as one aspect of a process for treating the recycled diluent of the butyl rubber polymerization process. So far as the inventors hereof are aware, no effective technique has been disclosed in the art for adding a complexing or buffering agent to the polymerization reaction mixture in order to eliminate the molecular weight depression and other problems associated with the presence of the polymerization poisons or activators noted above.
The use of aluminum alkyls to purify hydrocarbon diluents used in the polymerization of alpha-olefins is disclosed in British Pat. No. 920,513 (1963) issued to Eastman Kodak Company. Similarly, in British Pat. No. 1,298,909 (1972) issued to Monsanto Company, is disclosed the use of trialkyl aluminum compounds as a scavenger for water, oxygen and trace amounts of alcohol and other impurities in connection with a process for the Ziegler polymerization of olefins such as ethylene. While some of the trialkylaluminum compounds used in this Monsanto British Patent are employed in the present invention, the manner of their use in the reference and the polymerization process of the reference are distinct from the present invention. Similarly, in U.S. Pat. No. 3,257,332 issued June 2, 1966 to Ziegler et al, excess quantities of aluminum trialkyl are used to eliminate impurities in feedstock ethylene employed in an ethylene polymerization process. Also in U.S. Pat. No. 3,442,878 issued May 6, 1969 to Gippin, dihydrocarbon aluminum chlorides ae used to destroy impurities present in isoprene feedstock and in the hydrocarbon solvent used for isoprene polymerization.